A large number of methods and corresponding devices are available for capacitive measuring of the filling level in containers. Capacitive sensors with contactless operation for the recognition of the filling level of a dielectric medium inside containers having nonmetallic walls are known and are used, for example, in the chemical and pharmaceutical industries. DE 19949985 C2, for example, describes a sensor of this type. A construction consisting of a number of sensor fields arranged like a matrix is disclosed in DE 10008093 A1. A large number of capacitive sensors utilize stepped probes for determining the filling level (cf. DE 69001151 T2, DE 19938270 A1, DE 19757190 A1, DE 19721255 A1, or DE 19613813 C2) or other non-contactless methods (cf. DE 19754093 C2, DE 19516809 C1, or DE 10063557 A1). Moreover, methods for measuring a filling level using a plurality of capacitive sensors arranged next to one another along a filling path are known from DE 19916979 A1. U.S. Pat. No. 5,722,290 A describes the construction of a capacitive filling level meter having a ring oscillator. DE 69530863 T2 describes a filling level sensor, based on a transit time measurement, which can also be used as a linear path transducer. In EP 0760467 A1 too, there is described a method of measuring the filling level in a tube utilizing a capacitive method.
Methods and devices for the determination of density profiles in closed delivery devices are also known in the prior art. These methods include the class of electrical capacity tomography sensors (ECT). An example is shown in EP 0326266, in which appropriate reconstruction methods are also disclosed.
In U.S. Pat. No. 4,568,874 A, an arrangement is disclosed in which the presence of a liquid is determined with the aid of at least three electrode rings, the sensitivity being decreased by conductive deposits in the tube. The arrangement is used not for velocity measurement, but exclusively for density measurement. Determination of the dielectric property of the stream of transported material at the observation sites. (Kein Satz)
In U.S. Pat. No. 4,568,874 A, the method of “active guarding” is used, in which the auxiliary electrodes are supplied with the potential of the receiving electrodes in order to avoid unwanted effects or to shift the sensitivity range. This is a complicated method as regards circuit technology.
A device for capacitive measurement involving a reduction of leakage field interference is described in DE 4442711 A1. Here again, controlled auxiliary (active guarding) electrodes are used, which is again accompanied by a corresponding outlay in terms of circuit technology.
Many methods and relevant devices have been disclosed for measuring the velocity of flow of a stream of transported material. For instance, DE 4025952 A1 describes the measurement of the velocity of flow of fine-grain bulk goods in a pneumatic or hydraulic suspension by means of a contactless measuring method implementing capacitive sensors. Here, two transmitting electrodes lie spatially opposite to a sensor electrode on the outside of a measuring tube, an AC voltage being applied out of phase to the transmitting electrodes. Downstream or upstream thereof, again two transmitting electrodes and a sensor electrode are provided, the voltage supply here being of a different frequency. Using phase-sensitive rectifiers and signal processing by means of cross correlation, random fluctuations are detected and from these conclusions are drawn as to the velocity of flow. A similar measuring configuration having two pairs of electrodes is known from DE 3909177 A1. Just as in the aforementioned document, the detection and evaluation of random fluctuations of the mass flow, in this case coal dust, take place after high signal amplification with the aid of phase-sensitive rectifiers and a transit time correlator.
A measuring arrangement described in WO 01/65212 A1 uses two annular capacity sensors, located at a distance from one another and surrounding a flow pipe externally, having at least three electrodes each. Flow parameters are obtained by detection of capacity changes of the two sensors and cross correlation.
The problem of the spatial averaging of fluctuations is also known in the prior art. In EP 0108876 A1, a device is described in which spatial averaging is carried out by distribution of the electrodes in a pseudo-random manner along the tube, in order, on the one hand, to obtain sufficiently strong signals and, on the other hand, to reduce the averaging effect.
In the case of known flow measurements, a dielectric property of the transported material is determined at at least two sites in the direction of flow. It is necessary for this dielectric property to exhibit temporal fluctuations at each observation site. These fluctuations of the dielectric property can be of natural origin (e.g. concentration variations in the case of a turbulent flow) or can be introduced intentionally (e.g. injection of another medium into the stream of transported material).
On account of the necessary spatial extent of the measuring devices (electrodes), averaging of the fluctuations is carried out, which in turn leads to attenuation of the signal resolution.
A further difficulty consists in the fact that due to the formation of leakage fields a wide sensitivity range is produced which extends beyond the desired observation range of the measuring system. In order to obtain an adequate signal resolution, the distance between the two observation sites must be large. On the other hand, a large distance, in particular in the case of turbulent flow conditions, means that fluctuations during the movement from an observation site upstream to an observation site downstream are greatly changed (rheological disintegration of the fluctuation), whereby the signal strength of the correlation result decreases.
In particular, the use of a plurality of electrodes, as is necessary in the determination of a profile, further leads to high circuit complexity, since it is necessary to determine a number of measured values.
The complex arrangement of electrodes, electronics, and screening leads to complex mechanical layouts, which are in general difficult to incorporate in existing units.